Concrete is the material of choice in innumerable construction applications, because of its strength, longevity, and ease of use. Concrete is made from a mixture containing hydraulic cement, one or more aggregates, and water, and may also contain various additives that improve or alter the physical properties of the concrete, or that improve the processability of the mixture.
Fly ash, which is produced by the combustion of coal in electricity generating plants, is commonly added to concrete mixes. Fly ash can be added for a number of reasons, but is typically added for its cementitious properties. Some fly ash is itself a cement, and forms concrete when mixed with an aggregate and water. Most fly ash, however, is a natural pozzolan that has cementitious activity only when combined with a source of calcium oxide such as lime or cement. Because of this cementitious activity, fly ash can improve the compressive strength of concrete, and can also improve the resistance of concrete to chemical attack. Fly ash can also lower the cost of the concrete mix.
The concrete mix can also contain an air entraining agent to increase the air content of the finished concrete. Such air entraining agents are commonly included in mixes to entrain air in the finished concrete and thereby improve the resistance of the concrete to harmful freeze-thaw cycles.
Ashes other than fly ash have also been proposed for use in concrete. For example, U.S. Pat. No. 5,500,044 to Meade et al., and U.S. Pat. No. 3,957, 528 to Ott et al., disclose concrete mixes that contain the ash generated by a municipal waste incinerator. U.S. Pat. No. 5,346,548 to Mihta discloses a concrete mix containing the ash generated by rice husks. U.S. Pat. No. 611,621 to Frenchel discloses a concrete mix prepared from cement and wood ash.
None of the foregoing patents discloses the concrete mix of the present invention. Moreover, none of the ashes used in the above patents have been shown to improve the properties of concrete, and none have been commercially accepted in the concrete industry as a suitable additive for concrete.
It has been unexpectedly found, therefore, that ash generated by burning woody tissues, when combined with cement and coal ash, actually improves the physical properties of finished concrete. The woody tissue can be burned along with the coal to produce a mixture of woody tissue ash and coal ash, or it can be burned separately from the coal and subsequently combined with the coal ash and cement to obtain a suitable concrete mix. It has been unexpectedly found that woody tissue ash helps to entrain air in the finished concrete, and thereby improves the freeze-thaw characteristics of the concrete, while reducing (if not eliminating) the amount of artificial air entraining agent that must be included in the concrete mix.